1. Technical Field
The present invention relates to an image capturing optical system. More particularly, the present invention relates to a compact image capturing optical system which can be applied to an infrared optical system.
2. Description of Related Art
In recent years, with the popularity of mobile products with camera functionalities, the demand for compact photographing lenses is increasing, and the sensor of a conventional photographing camera is none other than CCD (Charge Coupled Device) or CMOS (Complementary Metal-Oxide Semiconductor Sensor). Furthermore, as advanced semiconductor manufacturing technology has allowed the pixel size of sensors to be reduced and compact photographing lenses have gradually evolved toward higher megapixels, there is an increasing demand for compact photographing lenses featuring better image quality.
A conventional compact photographing lens assembly for reducing the production cost mainly adopts a two-piece lens structure, as disclosed in U.S. Pat. No. 7,525,741. However, the ability for correcting the aberration by the two-piece lens structure cannot satisfy the requirements of the higher-level camera modules. But, the total track length cannot be reduced by arrange too many lens element.
For retaining high image quality and reducing total track length, another photographing lens assembly provides a three-lens structure, as disclosed in U.S. Pat. No. 7,443,613, wherein the three lens elements have a concave object-side surface and a concave image-side surface respectively, so that the refractive power of the lens elements be weakened by the shape of the lens elements while reducing the total track length of the photographing lens assembly difficulty.
Besides, the conventional photographing lens assembly and the conventional infrared optical lens assembly are made of different material. In general, the infrared optical lens assembly is made of germanium of others material, such as ZnSe, ZnS etc. Therefore, the conventional photographing lens assembly cannot be applied to an infrared optical system.